1. Field of the Invention
This invention relates to a process for subjecting materials to accelerated irradiance exposure factors that permit about a year's worth of representative weathering to be accumulated in a period from about 3 to about 10 days, under controlled weathering conditions that include several concurrent levels of temperature and relative humidity at very high levels of natural sunlight.
In the invention process, a solar concentrator [which may include a High Flux Solar Furnace (HFSF) and an Irradiance Redistribution Guide (IRG)] is used to obtain elevated levels(25-100.times.) of concentrated sunlight for accelerated testing of material samples.
When an IRG is used, it provides the unique capability of being able to modify (redistribute) the Gaussian-shaped beam from the HFSF into a more uniform profile on a sample exposure plane.
By adequately controlling sample temperatures and demonstrating that reciprocity relationships are obeyed (i.e., level of applied accelerated stresses does not change failure/degradation mode), this novel capability allows materials to be subjected to accelerated irradiance exposure factors of 25-100.times., thereby permitting a year's worth of representative weathering (in terms of natural sunlight exposure) to be accumulated in from about 3 to about 10 days.
2. Description of the Prior Art
U.S. Pat. No. 4,817,447 discloses a weathering chamber using lamps and sample temperature control using cooling air. Uniform sample irradiance at accelerated levels of up to 10 suns (within the UV bandwidth) appear attainable.
A test apparatus incorporating a mirror which rejects infrared is disclosed in U.S. Pat. No. 4,012,954. In the '954 patent, convective cooling air and a conductive substrate are also incorporated. However, although convective cooling is used, the air movement is not used to deliver humidity to the samples during exposure; rather, humidity is provided by floating the sample substrate in a water bath. Further, as in the case of U.S. Pat. No. 4,817,447, the '954 patent uses artificial light sources for exposure of the samples.
U.S. Pat. No. 3,686,940 discloses a water-cooled cylindrical mirror which rejects infrared radiation in an ultraviolet test apparatus. In the '940 patent, natural sunlight is not used.
A solar weathering device with control of sample temperature by cooling air is disclosed in U.S. Pat. No. 4,807,247. While this patent uses natural sunlight, a sample irradiance at accelerated levels of up to only 8 suns across the complete solar spectrum is employed.
U.S. Pat. No. 5,138,892 discloses accelerated light fastness testing of materials with xenon lamps and sample temperature control using air flow. Sample irradiance at accelerated UV levels of up to 8 suns (180 W/m.sup.2 between 300-400 nm) are attainable. This patent does not utilize natural sunlight in its testing of materials.
A weather test machine using xenon lamps and sample temperature and humidity control using air flow is disclosed in U.S. Pat. No. 5,646,358. Uniform sample irradiance at accelerated levels up to only 1-3 suns (within the UV bandwidth) are attainable. This patent does not utilize natural sunlight in its weather test machine.
U.S. Pat. No. 5,153,780 discloses a dish reflector and method for concentrating moderate solar flux uniformly on a target plane, said dish having stepped reflective surface characterized by a plurality of ring-line segments arranged about a common axis, each segment having a concave spherical configuration.
3. The Need for Capabilities Beyond the Prior Art
There is a need for devising facilities for ultra-accelerated natural sunlight exposure testing of materials and devices under controlled weathering conditions that include several concurrent levels of temperature and relative humidity at very high levels of natural sunlight. This need is associated with the desirability to be able to predict the in-service lifetimes of said materials and devices from correlations derived between such realistically accelerated test results and those obtained during normal use conditions. Further, there is a need to conduct these ultra-accelerated exposure tests at irradiance exposure factors of from about 25 to 100 suns, wherein the irradiance is highly uniform. Lastly, the need to conduct these ultra-accelerated natural sunlight exposure tests of materials and devices should exclude artificial light sources which invariably introduce uncertainties regarding realistic spectral content of the irradiance stress on samples being exposed. For example, the use of artificial light leads to unrealistic degradation mechanisms and failure modes of exposed materials caused by low wavelength (&lt;300 nm) photons that are not present in terrestrial solar spectra.